This invention relates to imaging techniques and apparatus in conjunction with total internal reflection at the boundary of an optically transparent material and more particularly to the use of such techniques and apparatus for detecting the presence, composition, quantity, and spatial distribution of substances on optically transparent substrates.
This invention relates to imaging of a biochip (also referred to as a gene chip, protein chip, microarray and others) useful in applications, which, for example, utilize local polarization changes detected in the respective parts of the emerging light beam. U.S. Pat. No. 5,633,724 to King, et al. (1997) describes the readout of a biochemical array using the evanescent field. The King et al patent focuses on fluorescent assays, using the evanescent field to excite fluorescent markers attached to the substances to be detected and analyzed. The attachment of fluorescent markers or other molecular tags to the substances to be detected on the surface requires an additional step in performing the measurement, which is not required in the current invention. The King et al patent further describes use of a resonant cavity to provide an evanescent field for exciting analytes.
The formation of an array of biologically or chemically active spots on the surface of a substrate for identifying constituents in test material brought into contact with the array is well known. Typically, such processes require spots of, for example, oligonucleotides, DNA clones, antibodies, peptides, receptors, enzymes, inhibitors, etc. which are processed to exhibit fluorescence, electroluminescence, current change, voltage changexe2x80x94etc. for providing a detectable signature for the presence of constituents in the material being tested.
In accordance with the principles of this invention, light from a light source member providing an extended, polarized light beam, is directed through a transparent substrate and undergoes total internal reflection (TIR) at the surface of the substrate by a single reflection within the TIR member. Total Internal Reflection is described in: M. Born, and E. Wolf, xe2x80x9cPrinciples of Opticsxe2x80x9d, 6th ed., pp 47-51, Pergamon Press, Oxford, 1991. The reflected light is detected by a polarization-sensitive, two-dimensional array detector or other type of detector. The changes of the local polarization state in the beam""s cross-section caused by the total internal reflection are employed to obtain information about the presence and composition of substances in an array of substances on the substrate surface for each point of the surface. In accordance with one aspect of the invention, the light-generating element within the light source member is a quasi-monochromatic light source of moderate bandwidth.
In one embodiment, the light-generating element within the light source member is an Arc lamp. The light from the light source member is directed through an internal reflection member to reflect off a specimen. The total internal reflection at any point within the cross-section of the light beam causes a phase shift between the light component polarized in the plane of incidence and the component polarized perpendicular to the plane of incidence. The reflected light is detected by a polarization-sensitive detector such as a two dimensional array detector and the signal from this detector is then processed in a computer to provide two-dimensional information about substances on the surface of the specimen. Spatially distributed changes in polarization state in the cross-section of the reflected beam are indicative of the substances in the specimen in the location in the specimen array corresponding to a position in the detector. The apparatus and method is especially adapted for imaging material in an aqueous solution. It is furthermore particularly suited for detecting attachment and detachment of analytes to a two-dimensional biomolecular array positioned on the total internal reflection member as part of a molecular thin film system. In various applications a plurality of discrete specimen spots are presented in an array, where the method and apparatus will image the array so as to distinguish each of the discrete specimen spots. Fluorescence or molecular tagging is not necessary or practical for use in this invention.
Further, in accordance with the principles of this invention, the apparatus disclosed in the above-identified parent application provides an image of an entire array on a biochip or if desired a portion of the entire array.
In further embodiments the invention relates to a device having a removable portion, which in preferred embodiments is disposable or reusable, the removable portion defining a cassette having an optical element capable of receiving polarized light and directing it to a TIR surface to create an evanescent field in which the material to be imaged is positioned, and then upon a single reflection, reflecting the light beam to exit the optical element; the cassette also having a mounting structure to enable it to be mounted in the processing portion of the device that produces the polarized beam and receives it after reflection, for analysis. The cassette is designed to mount on the processing portion for operative interaction with the light beam as described herein. The cassette may be provided with a microarray of spots already in place on the optical element or it may be configured to receive a separate slide on which a microarray of spots have been placed. The cassette may be either disposable or reusable. When a separate slide is used in conjunction with the optical element, it is mounted on the optical element with an index matching fluid so that the surface of the slide on which the microarray is placed is the TIR surface.
In one aspect of the invention the optical element is a prism. A further specific aspect of the invention comprises the use of an optical element having gratings on it to control the direction of the incoming beam to enable TIR to occur instead of using a prism.
One of the goals is to define a cassette, which is sufficiently inexpensive that it is practical to dispose of it after a single use. While prisms can be used and reused, they are expensive. The aspect of the invention that uses gratings provides for a much less expensive cassette.
Nevertheless the use of gratings on an optical element to enable TIR to occur may be implemented broadly for any apparatus or method as an alternative to use of a prism. Also, certain other techniques are made available by use of gratings such as electronically alterable gratings and the use of holographic gratings.